I'm using WindowsBuilder Pro newly. First, I plugged in to eclipse which is stand on my Xubuntu Virtual Machine. After a few tries, i decided to setup on windows either. But, although that i applied the same procedure to setup, when i try to switch design mode of a JFrame i get the error below:

Internal Error
encountered unexpected internal error.
This could be caused by a bug or by a misconfiguration issue, conflict, partial update, etc.
java.lang.NoClassDefFoundError: net/sf/cglib/proxy/Callback
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Stack trace:
java.lang.ClassNotFoundException: net.sf.cglib.proxy.Callback
at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:513)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:429)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:417)
at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.loadClass(DefaultClassLoader.java:107)
at java.lang.ClassLoader.loadClass(Unknown Source)
at org.eclipse.wb.internal.core.utils.state.EditorState.getBroadcast(EditorState.java:130)
at org.eclipse.wb.internal.core.parser.JavaInfoParser.<init>(JavaInfoParser.java:217)
at org.eclipse.wb.internal.core.parser.JavaInfoParser.<init>(JavaInfoParser.java:209)
at org.eclipse.wb.internal.core.parser.JavaInfoParser.parse(JavaInfoParser.java:150)
at org.eclipse.wb.internal.core.editor.DesignPage.internal_refreshGEF(DesignPage.java:537)
at org.eclipse.wb.internal.core.editor.DesignPage.access$9(DesignPage.java:525)
at org.eclipse.wb.internal.core.editor.DesignPage$8$1.run(DesignPage.java:452)
at org.eclipse.swt.widgets.Synchronizer.syncExec(Synchronizer.java:180)
at org.eclipse.ui.internal.UISynchronizer.syncExec(UISynchronizer.java:150)
at org.eclipse.swt.widgets.Display.syncExec(Display.java:4683)
at org.eclipse.wb.internal.core.editor.DesignPage$8.run(DesignPage.java:449)
at org.eclipse.jface.operation.ModalContext.runInCurrentThread(ModalContext.java:464)
at org.eclipse.jface.operation.ModalContext.run(ModalContext.java:372)
at org.eclipse.jface.dialogs.ProgressMonitorDialog.run(ProgressMonitorDialog.java:507)
at org.eclipse.wb.internal.core.editor.DesignPage.internal_refreshGEF_withProgress(DesignPage.java:468)
at org.eclipse.wb.internal.core.editor.DesignPage.internal_refreshGEF(DesignPage.java:418)
at org.eclipse.wb.internal.core.editor.UndoManager.refreshDesignerEditor(UndoManager.java:381)
at org.eclipse.wb.internal.core.editor.UndoManager.activate(UndoManager.java:90)
at org.eclipse.wb.internal.core.editor.DesignPage.handleActiveState_True(DesignPage.java:266)
at org.eclipse.wb.internal.core.editor.DesignPage.handleActiveState(DesignPage.java:244)
at org.eclipse.wb.internal.core.editor.multi.DefaultMultiMode.showPage(DefaultMultiMode.java:125)
at org.eclipse.wb.internal.core.editor.multi.DefaultMultiMode$1.widgetSelected(DefaultMultiMode.java:63)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:240)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1053)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1077)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1062)
at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:774)
at org.eclipse.swt.custom.CTabFolder.setSelection(CTabFolder.java:2746)
at org.eclipse.swt.custom.CTabFolder.onMouse(CTabFolder.java:1433)
at org.eclipse.swt.custom.CTabFolder$1.handleEvent(CTabFolder.java:257)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1053)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4165)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3754)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2701)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2665)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2499)
at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:679)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:668)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:123)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:344)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:622)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:577)
at org.eclipse.equinox.launcher.Main.run(Main.java:1410)
at org.eclipse.equinox.launcher.Main.main(Main.java:1386)
Full context stack trace:
java.lang.NoClassDefFoundError: net/sf/cglib/proxy/Callback
at org.eclipse.wb.internal.core.utils.state.EditorState.getBroadcast(EditorState.java:130)
at org.eclipse.wb.internal.core.parser.JavaInfoParser.<init>(JavaInfoParser.java:217)
at org.eclipse.wb.internal.core.parser.JavaInfoParser.<init>(JavaInfoParser.java:209)
at org.eclipse.wb.internal.core.parser.JavaInfoParser.parse(JavaInfoParser.java:150)
at org.eclipse.wb.internal.core.editor.DesignPage.internal_refreshGEF(DesignPage.java:537)
at org.eclipse.wb.internal.core.editor.DesignPage.access$9(DesignPage.java:525)
at org.eclipse.wb.internal.core.editor.DesignPage$8$1.run(DesignPage.java:452)
at org.eclipse.swt.widgets.Synchronizer.syncExec(Synchronizer.java:180)
at org.eclipse.ui.internal.UISynchronizer.syncExec(UISynchronizer.java:150)
at org.eclipse.swt.widgets.Display.syncExec(Display.java:4683)
at org.eclipse.wb.internal.core.editor.DesignPage$8.run(DesignPage.java:449)
at org.eclipse.jface.operation.ModalContext.runInCurrentThread(ModalContext.java:464)
at org.eclipse.jface.operation.ModalContext.run(ModalContext.java:372)
at org.eclipse.jface.dialogs.ProgressMonitorDialog.run(ProgressMonitorDialog.java:507)
at org.eclipse.wb.internal.core.editor.DesignPage.internal_refreshGEF_withProgress(DesignPage.java:468)
at org.eclipse.wb.internal.core.editor.DesignPage.internal_refreshGEF(DesignPage.java:418)
at org.eclipse.wb.internal.core.editor.UndoManager.refreshDesignerEditor(UndoManager.java:381)
at org.eclipse.wb.internal.core.editor.UndoManager.activate(UndoManager.java:90)
at org.eclipse.wb.internal.core.editor.DesignPage.handleActiveState_True(DesignPage.java:266)
at org.eclipse.wb.internal.core.editor.DesignPage.handleActiveState(DesignPage.java:244)
at org.eclipse.wb.internal.core.editor.multi.DefaultMultiMode.showPage(DefaultMultiMode.java:125)
at org.eclipse.wb.internal.core.editor.multi.DefaultMultiMode$1.widgetSelected(DefaultMultiMode.java:63)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:240)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1053)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1077)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1062)
at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:774)
at org.eclipse.swt.custom.CTabFolder.setSelection(CTabFolder.java:2746)
at org.eclipse.swt.custom.CTabFolder.onMouse(CTabFolder.java:1433)
at org.eclipse.swt.custom.CTabFolder$1.handleEvent(CTabFolder.java:257)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1053)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4165)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3754)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2701)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2665)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2499)
at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:679)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:668)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:123)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:344)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:622)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:577)
at org.eclipse.equinox.launcher.Main.run(Main.java:1410)
at org.eclipse.equinox.launcher.Main.main(Main.java:1386)
Caused by: java.lang.ClassNotFoundException: net.sf.cglib.proxy.Callback
at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:513)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:429)
at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:417)
at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.loadClass(DefaultClassLoader.java:107)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 56 more

I added the cglib.jar as referenced library to my project configuration, but still i got the same issue.

I have no idea.
I can only repeat error message - there is some installation problem.
Eclipse error log (workspace/.metadata/.log) may help, may be there are OSGi messages about problems with installed bundles.
But easier would be try to install WB again.